The present invention relates to a glass substrate having a transparent conductive film for a functional element. More particularly, the invention relates to a glass substrate which comprises a base glass plate having a smooth surface, a continuous alkali barrier film, and a relatively thick, transparent, conductive film overlying the barrier film and in which the transparent conductive film is prevented from developing hair cracks by forming, between the base glass plate and the barrier film, an undercoat for imparting roughness which has a thickness of 150 xc3x85 or larger and is made of a crystalline metal oxide. This glass substrate having a transparent conductive film is especially useful as a substrate for an amorphous silicon solar cell.
Amorphous silicon (hereinafter abbreviated as xe2x80x9ca-Sixe2x80x9d) solar cells are recently used in increasingly broadening applications, ranging from civilian goods such as electronic calculators and watches to electric power supply. In general, a-Si solar cells have a multilayer constitution comprising base glass plate/transparent conductive film/a-Si film/metal electrode film. Sunlight incident on such a solar cell passes from the base glass plate side through the transparent conductive film and then enters the a-Si film. The base glass plate and the transparent conductive film are hence required to have a high transmittance.
Such solar cells, when used for electric power supply, are mounted on the roofs or walls of houses, buildings, etc. and need to have a large area (e.g., 900 mmxc3x97600 mm). Consequently, these solar cells frequently employ an inexpensive soda-lime glass (alkali-containing glass) produced by the float process as the base glass plate. In this case, a continuous alkali barrier film is formed on the surface of the base glass plate for the purpose of preventing the alkali components present inside the glass from dissolving away and coming into the coating films to denature or damage the same. A thin film of SiO2 (silicon oxide) is generally used as the continuous alkali barrier film.
Consequently, this kind of glass substrate has a constitution with two films, i.e., base glass plate/SiO2, film/transparent conductive film. The transparent conductive film used is a metal oxide such as ITO (indium-tin oxide), SnO2 (tin oxide) or ZnO (zinc oxide). Those metal oxide films are deposited by a method such as CVD (chemical vapor deposition), sputtering or vacuum vapor deposition. For use in electric power supply,- SnO2 films deposited by CVD are frequently employed because these films are inexpensive and highly suitable for mass production and have a higher adhesion strength than SnO2 films deposited by sputtering or vacuum vapor deposition.
In a-Si solar cells for electric power supply, it is important for the transparent conductive film to have reduced electrical resistance because these cells have a large panel structure. In particular, the transparent conductive film made of SnO2, which is inexpensive, is made to have reduced electrical resistance as a whole by doping the SnO2 with an appropriate impurity and increasing the thickness thereof, because SnO2 has a higher resistivity than ITO. Specifically, the thickness of the transparent conductive SnO2 film should be 5,000 xc3x85 or larger.
In forming an SnO2 film on a glass substrate by CVD, the substrate temperature is maintained at around 650xc2x0 C. during the film deposition. Also in the case of film deposition by sputtering or vacuum vapor deposition, the substrate is frequently heated in order to increase the adhesion strength of the film. Glass substrates constituted of a soda-lime glass plate and a two-layer coating formed by successively depositing a continuous alkali barrier film of SiO2 and a transparent conductive film of SnO2 in this order were subjected to an accelerated test in a high-temperature and high-humidity atmosphere (e.g., 80xc2x0C., 100% RH). As a result, it was observed that the transparent conductive films having a thickness of 5, 000 xc3x85 or larger developed hair cracks over the entire surface.
A close examination with Scanning Electron Microscope (SEM) revealed that these hair cracks had a width of tens of micrometers (typically, about 20 xcexcm), and that in these hair crack areas, the SnO2 film had completely peeled off to expose the SiO2 film.
An object of the invention is to provide a glass substrate which has a transparent conductive film comprising SnO2 and in which the transparent conductive film is prevented from developing hair cracks even when it has a thickness as large as 5,000 xc3x85 or above.
The present invention provides a glass substrate having a transparent conductive film, that is used to laminate a film for a functional element thereon to form a laminate structure. This glass substrate having a transparent conductive film comprises, as shown in FIG. 1, an alkali-containing base glass plate 10 having a smooth surface and, formed thereon in this order, an undercoat 12 for imparting roughness which has a thickness of 150 xc3x85 or larger and is made of a crystalline metal oxide, a continuous alkali barrier film 14 made of SiO2 and having such a small thickness as to reflect the surface roughness of the undercoat, and a transparent conductive film 16 made of SnO2 doped with one or more appropriate impurities and having a thickness of 5,000 xc3x85 or larger. The transparent conductive film has an adhesion strength of 37 mN or higher as measured in accordance with xe2x80x9cAdhesion Test for Thin Film Deposited on Glass Substratexe2x80x9d as provided for in JIS R3255-1997.